Commutators, being rotary switches used with DC electric motors, comprise multiple electrically conductive segments arranged into a cylinder or plane and anchored into a non-conducting, typically phenolic resin, moulding compound. Each segment is physically separated and electrically isolated from those adjacent to it, so that an electrical, typically carbon, brush passing along the outer surface thereof will form a conductive path only with the segment (or segments) in contact with it at any given instant.
FIGS. 1 to 4 show one known planar commutator 1. This commutator comprises a circular phenolic electrically non-conductive base 2 having a central aperture 3 for a motor shaft, and a plurality of electrically conductive segments 4 supported by the base 2. Each segment 4 includes a copper inner layer 5 from which a tang 6 extends for connection to a winding of the rotor, and a graphite brush-contacting outer layer 7 which is fixed to the copper inner layer 5.
The inner layer 5 of the segment is formed with a number of barbs 8, in this case being three. The barbs project at a slight angle from edges of the inner layer, and locate at edges of the electrically non-conductive base 2. While this arrangement does provide a particularly secure and robust attachment, it prevents the commutator from being miniaturised due to the process of forming the radially inner barb. This poses a size limitation related to the number of segments. Omitting the inner barb leads to an unstable connection between the segment and the base which, during use in harsh vibrational environments, can result in loosening.
To attach the graphite outer layer 7 of the segment to the copper inner layer 5, soldering is typically used. However, using X-ray imaging, it has been found that excess flux and air is trapped between the two layers 5, 7 during attachment, thus resulting in a non-uniform and weaker connection.
With the copper inner layer 5 connected to the phenolic base 2 and the graphite brush-contacting outer layer 7 connected to the inner layer 5, the segments of the commutator are then formed by cutting, typically with a circular saw being drawn diagonally across the commutator. However, it has been found that the outer perimeter edge of the phenolic base 2 is often chipped by the cutting device as the cutting device is drawn out and away from the base. This results in a higher scrap rate. The chipped area weakens the commutator base, and thus cracking can more easily occur, accelerating brush wear and impacting the longevity of the motor. Also, the chipped area may prevent a mould from fully sealing around the perimeter edge, thus allowing armature overmould plastic to enter into the commutator slot, which shortens the life of the commutator.